enigma

The definition of enigma is something or someone that is puzzling or a riddle.The word "enigma" first appeared in print in 1449. Its origins remain as mysterious as the word itself. It is likely to derive from the Latin ænigma, meaning a riddle, or the Greek, ainigma, which derives from the word ainissesthai, "to speak obscurely, speak in riddles."

An example of an enigma is the question “Which came first, the chicken or the egg?”